CHAPTER V 



H-c 



HEXAGONAL SYSTEM 



THE hexagonal system includes all those crystals which may be 

 referred to four axes, three of which, the lateral axes, lie in one 



plane, the equatorial 

 plane. They are equal 

 and interchangeable 

 and inclined 60 to each 

 other. They are all 

 designated by the let- 

 a. ter a. The order of + 



and extremities are 

 as shown in Fig. 149. 

 The fourth or c axis is 

 the vertical axis and is 

 at right angles to the a 

 axes and not inter- 

 changeable with them. 

 It may be either longer 

 or shorter. 



In the hexagonal system the parameters have four terms, 

 and are written in the following order, na : : pa 2 : a 3 : me. As 

 all three lateral axes are in ' 



n 



the equatorial plane and all 

 faces, except the base, in- 

 tersect this plane in straight 

 lines, two lateral axes and 

 the c axis will fix the in- 

 clination of any face, for 

 the straight line dd', Fig. 

 150, is fixed by the inter- 

 cepts on the axes ai and a 2 . 

 The value of the intercept 

 a 2 , or the coefficient p in 

 the general set of parame- 

 ters, when one intercept is FIG. 150. 



84 



C 



FIG. 149. 



